1. Field of the Invention
The present invention relates to a novel zirconium-based hydrogen storage alloy useable for negative electrodes for secondary batteries. More particularly, the present invention relates to a single-phase, Zr-based alloy which has a C-14 hexagonal structure and improved temperature and current density dependencies of discharge capacities, and of which alloying elements and composition are specifically modified so as to be suitable for practical nickel-metal hydride (Ni--MH) secondary batteries.
2. Description of the Prior Art
U.S. patent application Ser. No. 08/012,408, filed Feb. 2, 1993 describes two groups of zirconium-based hydrogen storage alloys which are useful as negative electrodes for rechargeable batteries. The first group is represented by the formula:. EQU Zr-based metal hydrides+Mx
wherein M is a light rare earth metal selected from the group consisting of La, Nd, and Mm; 0&lt;x&lt;0.1; and the Zr-based metal hydrides means that the metal hydrides are mainly in a Zr-based Laves phase such as ZrCrNi and Zr(V.sub.0.33 Ni.sub.0.67).sub.2.4. The second group is represented by the formula: EQU ZrCr.sub.1+y Ni.sub.1+z
wherein 0.ltoreq.y&lt;0.2, and 0.ltoreq.z.ltoreq.0.2, provided that y and z cannot be zero concurrently.
In the last few years, popularization of various cordless appliances such as video cameras, laptop computers and cellular telephones has increased the production of sealed nickel/cadmium (Ni--Cd) batteries. This has caused a shortage in cadmium supply and a rapid increase in the price of cadmium to more than four times its former cost. In addition, failure to recover spent Ni--Cd batteries is causing serious environmental problems since cadmium, a toxic heavy metal, is a source of pollution.
Since the mid-1980s, much research has been conducted on the replacement of cadmium with other elements. Nickel-metal hydride [Ni--MH] batteries using a hydrogen storage alloy in place of cadmium have been developed. See, U.S. Pat. Nos. 3,874,928 to will; 4,004,943 to Boter; and 4,214,043 to Deutekom; and Japanese Patent Publication No. 62-296,365 Al. Since 1991, such Ni--MH secondary batteries have been commercialized.
In the Ni--MH batteries, the electrode reactions occur as follows. During discharging, hydrogen atoms released from the hydrogen storage alloy combine with hydroxide ions (OH.sup.-) in an electrolyte solution to form water, with a concurrent flow of electrons through an external circuit to a positive electrode. During charging, water in the electrolyte solution is decomposed into a hydrogen (H.sup.+) and a hydroxide ion (OH.sup.-). The hydrogen ion (H.sup.+) accepts an electron to form atomic hydrogen which in turn combines with and is stored by the hydrogen storage alloy in its atomic state. The hydroxide ion (OH.sup.-) remains in the electrolyte solution. These Ni--MH batteries have been made by taking advantage of the reversibility of hydrogen storage alloys that rapidly and stably absorb and release a large amount of hydrogen in an alkaline solution. It has been found that such Ni--MH batteries have a 1.5-2 times larger cell capacity and a 2 times or faster charge rate than conventional nickel/cadmium batteries, with almost comparable cell characteristics, such as voltage, rate capability, overcharge protection, and charge retention.
Hydrogen storage alloys may have very different properties depending on their types. Only hydrogen storage alloys having a plateau pressure of about 1 to 0.01 atm at about 0.degree. to 40.degree. C. can be used as negative electrodes for secondary batteries. See, Proc. Int. Sym. Hydrides for Energy Storage, Geilo, Norway, pp. 261 (1977). Representative examples of hydrogen storage alloys which can be used in secondary batteries, include La--Ni based alloys [See, U.S. Pat. No. 4,487,817 to Willens et al.]; Mm(misch metal)--Mn--Ni--Co--Al based alloys [See, Japanese Patent Publication Nos. 61-132,501 Al and 61-214,360 Al]; Ti--V--Ni--Cr based alloys [See, U.S. Pat. No. 4,551,400 to Sapru et al.]; Zr--V--Ni based alloys [See, Y. Moriwaki at al., J. Less-Common Metals, Vol. 172-174, pp. 1219-1226 (1991)]; and Zr--Cr--Mn--Ni based alloys [See, Y. Moriwaki et al., J. Less-Common Metals, Vol. 172-174, pp. 1211-1218 (1991)].
Among the above alloys, however,. the La--Ni based alloys suffer from the disadvantage that they have a very sharply decreasing electrode capacity in alkaline electrolytes during charge/discharge cycles. See, A. H. Boonstra, et al., J. Less-Common Metals, Vol. 161, pp. 193 (1989), and idem, Vol. 155, pp. 119-131 (1989). This drawback, so-called "degradation," can be reduced somewhat by electroless plating with copper of the surface of the alloys in order to extend the lifetime of the La--Ni based alloy electrodes. See, H. Ishikawa et al., J. Less-Common Metals, Vol. 107, pp. 105-110 (1985). This electroless plating contributes somewhat to the extension of the lifetime of electrodes, but can create another source of environmental pollution because the processes involve the use of an aqueous solution which produces chlorine gas.
Another approach to overcoming the disadvantage involves the recent development of Zr-based hydrogen storage alloys. These have a larger hydrogen absorbing capacity and a longer cycle life than conventional hydrogen storage alloys. Examples of such Zr-based hydrogen storage alloys include Zr--V--Ni based alloys and Zr--Cr--Mn--Ni based alloys, which have a C-15 cubic structure (f.c.c.: body centered crystal). See, To Sakai et al., J. Less-Common Metals, Vol. 172-174, pp. 1175-1184 (1991), and idem, Vol. 180, pp. 37 (1992).
In general, hydrogen storage alloys to be used as electrode materials for secondary batteries should have excellent electro-chemical properties such as discharge capacity, lifetime, low temperature discharge rate, charge/discharge rate capability, and self-discharge rate. Also, hydrogen storage alloys to be used as electrode materials for secondary batteries are required to have the hydriding properties of appropriate plateau pressure, large hydrogen absorbing capability, fast hydriding rate, and long cyclic life. It has been found that Zr-based hydrogen storage alloys almost satisfy the requirements for electrode materials for secondary batteries mentioned above, but have quite low equilibrium hydrogen pressures and inclined plateau pressures. Therefore, those alloys have been found deficient as electrode materials for secondary batteries.
Consequently, it has been desired to provide a Zr-based hydrogen storage alloy having improved performances in terms of temperature and reaction rate dependencies of discharge capacity with a increased discharge capacity and cycle behaviors such as cycle life.
We have found that a single phase, Zr-based hydrogen storage alloy having specific alloying elements and composition, wherein Zr is partially replaced with Ti or Hf, and wherein Cr is partially replaced with one or more alloying elements selected from the group consisting of Mn, Fe, Co and V, has the C-14 hexagonal structure and exhibits improved, performance in terms of temperature and current density dependencies of discharge capacities, as compared with conventional Zr-based alloy materials, with improved discharge capacity and cycle behavior.